FIG. 1A is a high level block diagram that illustrates some of the components of an exemplary conventional cardiac stimulation device 102, which can also be referred to as a pacing device, a pacemaker, or more generally, as an implantable medical device (IMD). The cardiac stimulation device 102 includes a pulse generator 104 that is configured to selectively output an electrical stimulation pulse. The pulse generator 104 is shown as being a part of an integrated circuit (IC) 106 that also includes switches 108_2, 108_3 . . . 108_N. The cardiac stimulation device 102 is also shown as including a plurality of electrode terminals 110_1, 110_2, 110_3 . . . 110_N, each of which is coupleable to a separate electrode of a lead 122. The components of the cardiac stimulation device 102 shown in FIG. 1A can all be considered components of a single pacing channel. One of ordinary skill in the art will appreciate that a cardiac stimulation device can include multiple pacing channels, as well as other components not discussed herein.
The electrodes of the lead 122, which are labeled 124a, 124b, 124c . . . 124x, are in contact with body tissue 132, which can also be referred to as patient tissue. In FIG. 1A, the electrode terminal 110_1 is a pace output terminal, and the remaining electrode terminals labeled 1102, 110_3 . . . 110_N are pace return terminals. The body tissue 132 can be, e.g., cardiac tissue within or outside one of the cardiac chambers, such as the left ventricle, right ventricle, left atrium and right atrium, but is not limited thereto. In this FIG., and the other FIGS. discussed herein, the resistor symbols shown within the patient tissue block 132 (and other patient tissue blocks) are representative of the resistances associated with the patient tissue.
Also shown in FIG. 1A are pace return capacitors, each labeled C_PACE_RTN, which are also known as direct current (DC) blocking capacitors. Each pace return capacitor is used to achieve charge neutrality for its corresponding electrode, thereby preserving lead integrity and preventing patient tissue damage. A lack of charge neutrality would result in a DC current flowing through patient tissue, which is undesirable. Advantageously, each pace return capacitor prevents DC signals from flowing through an electrode and corresponding patient tissue.
Conventionally, a separate pace return capacitor C_PACE_RTN is in series with each separate cathodic electrode 124b, 124c . . . 124x. Accordingly, if the conventional design of FIG. 1A is used, as the number of pacing sites and pacing electrodes increases, the number of pace return capacitors also increases, which increases the cost and potentially the size of the cardiac stimulation device. This is one disadvantage of the conventional design in FIG. 1A, especially considering that each pace return capacitor is relatively expensive and relatively large compared to many of the other components of a cardiac stimulation device.
As shown in FIG. 1A, the switches 108_2, 108_3 . . . 108_N are used to selectively couple one of the pace return capacitors C_PACE-RTN to a ground reference. Conventionally, as shown in FIG. 1A, each of the pace return capacitors C_PACE-RTN is hardwired between a respective pace return terminal (110_2, 110_3 . . . 110_N) and a respective cathodic electrode (124b, 124c . . . 124x), whether or not the cathodic electrode is used. Accordingly, with the conventional design, if certain cathodic electrode(s) is/are not used, the pace return capacitor(s) C_PACE-RTN hardwired to the unused cathodic electrode(s) are not used, and thus only add cost and size to the cardiac stimulation device without adding any useful functionality. This is another disadvantage of the conventional design in FIG. 1A.
Where multi-site pacing and/or bi-ventricular pacing is implemented, charge neutrality should be maintained for each electrode individually. However, where multi-site pacing and/or bi-ventricular pacing is implemented, separate pacing pulses can occur close enough in time to one another, such that a pace return capacitor does not have sufficient time to discharge between the pacing pulses, which can prevent charge neutrality from being achieved. This is illustrated with reference to FIGS. 1B and 1C. Referring to FIG. 1B, at a first point in time, the switch 108_2 is closed and the pulse generator 106 outputs a first pacing pulse, resulting in a charge on the pace return capacitor C_PACE-RTN coupled between the electrode 124b and the pace return terminal 110_2. Such a charge is represented in FIG. 1B by “−−−” and “+++” symbols. The line 111 in FIG. 1B illustrates the electrical signal path associated with delivery of the first pacing pulse. Referring now to FIG. 1C, at a second point in time, the switch 108_2 is opened, the switch 108_3 is closed and the pulse generator 106 outputs a second pacing pulse, resulting in a charge on the pace return capacitor C_PACE-RTN coupled between the electrode 124c and the pace return terminal 110_3 as illustrated by “−−−” and “+++” symbols. The line 112 in FIG. 1B illustrates the electrical signal path associated with delivery of the second pacing pulse. Here, however, because the time between the first and second pacing pulses was too short, the pace return capacitor C_PACE-RTN coupled between the electrode 124b and the pace return terminal 110_2 did not yet have time to fully discharge as illustrated by the “−” and “+” symbols. Disadvantageously, this can result in the discharging of a pace return capacitor during pacing and/or discharging from an unwanted signal path (represented as a dotted lined signal path 113 in FIG. 1C) associated with a parasitic diode (represented by the dotted lined diode 114 in FIG. 1C) which is/are intrinsic to any integrated circuit. If the current that charges a pace return capacitor C_PACE-RTN comes from one electrode but returns through another path, electrodes will be out of balance. If this continues for multiple pacing pulse, a DC current will flow in multiple electrodes, potentially leading to tissue damage. Conventionally, to avoid these potential problems, the amount of charge on a given pace return capacitor could be limited, pacing pulse amplitudes could be limited, pacing pulse widths could be limited and/or inter-pulse delays could be limited. In other words, conventionally there are pacing pulse limitations that should be followed to avoid unwanted discharge paths, which limits the flexibility of the conventional design.